Kinematic resonator support

ABSTRACT

The subject invention relates to a laser system where the length stability of the resonant cavity is important. Three unique mounting assemblies are disclosed to improve the stability of the cavity. The first assembly relates to a kinematic support for mounting an Invar bar. The second assembly defines a once-piece gimbal pivot ring designed to mount laser mirrors. The final aspect of the invention includes a mounting assembly designed to automatically compensate for changes in length of an Invar support bar due to temperature variations which could alter the cavity length of the laser.

TECHNICAL FIELD

The subject invention relates to a laser system where the lengthstability of the resonant cavity is critical. Three mounting assembliesare disclosed for enhancing the stability of the optical path length ofthe laser resonator.

BACKGROUND OF THE INVENTION

The need to provide a resonate cavity of stable optical length forcertain laser applications is well known. For example, stable resonatorconfigurations have been used to enhance single frequency operation andmode-locking. In this context, a stable resonator is intended to meanthat the length of the resonant cavity (measured along the laser beambetween the end mirrors) is held constant.

A number of approaches have been taken in the prior art to improve thestability of the resonator. For example, it is well known that thestability of the cavity can be improved if all optical components aremounted on a bar having a relatively low thermal coefficient ofexpansion. One suitable material is Invar. In use, the bar is connectedto the frame or laser housing through some form of kinematic mounting. Agood kinematic mounting will minimize the number of contact points withthe bar while removing any degrees of freedom of motion of the bar withrespect to the frame. In this manner, any force placed on the frame,which could misalign the optical elements, will not be transferred tothe bar. In accordance with one aspect of the subject invention, animproved kinematic mounting for an Invar bar is disclosed.

Although materials such as Invar have a relatively low coefficient ofthermal expansion, the length of the Invar bar will change to somedegree in response to variations in temperature. Variouselectromechanical servo mechanisms have been developed to compensate forgross deviations in length due to temperature. Another aspect of thesubject invention is to provide a new and improved means forautomatically compensating for small variations in the change in lengthof a support bar such that the total length of the resonant cavity canbe maintained constant despite small environmental temperature changes.

The mirrors at the end of a resonant cavity are typically mounted in anadjustable support. A standard support consists of two plates which canbe rotated with respect to each other about two pivot lines. Theadjustments comprise a pair of screws for making this adjustment. Theproblem with this standard mirror mount is that as the mirror isrotated, the mirror face moves with respect to the beam path, changingthe length of the resonant cavity. In the final aspect of the subjectinvention, an improved gimbal mirror mount is disclosed which overcomesthis difficulty.

Accordingly, it is an object of the subject invention to provide a newand improved laser having enhanced stability.

It is another object of the subject invention to provide an improvedkinematic mounting assembly for a laser.

It is still another object of the subject invention to provide a new andimproved kinematic mounting assembly for connecting a mirror supportstructure or bar to a frame of a laser.

It is still a further object of the subject invention to provideimproved mirror mounting assemblies.

It is still another object of the subject invention to provide improvedmirror mounting assemblies which are designed to maintain the length ofthe resonant cavity in the laser.

It is another object of the subject invention to provide a gimbalassembly for pivotally mounting a laser mirror.

It is still a further object of the subject invention to provide amounting assembly for a mirror which automatically compensates forchanges in the length of a support bar due to temperature.

SUMMARY OF THE INVENTION

In accordance with these and many other objects, the subject inventionprovides for a plurality of improved support mechanisms for enhancingthe stability of a laser resonator. The first improvement includes akinematic assembly for mounting an Invar bar to a laser frame. Thekinematic assembly includes a pair of brace members for receiving thebar. The brace members function to support the bar while allowinglimited longitudinal and rotational movement thereof. A clamp member issecurely mounted to the bar next to one of the brace members. This clampmember is then connected to the associated brace member through theinteraction between a cone and an elongated slot. This interfaceprovides the minimum number of constraints to just constrain all degreesof freedom of motion of the bar. The various mirror assemblies of theresonator can then be mounted to the bar for stable performance even ifthe frame of the laser is subjected to an unwanted force.

The subject invention further includes a new and improved mountingparticularly suited for supporting the end mirrors of the laser cavity.The mount includes a one piece cylindrical gimbal ring having fourflexure areas spaced 90° apart around the circumference thereof. Thestraight line between each pair of opposed flexure areas defines a pivotaxis. The two pivot axes intersect at the center of the ring. One endface of the ring is mounted to a fixed member connected to the bar. Amirror plate, having a mirror holder, is connected to the other end faceof the ring. In a preferred embodiment, the mirror holder locates thepoint on the surface on the mirror where the laser beam strikessubstantially at the point of the intersection of the two pivot axes.Adjustments of the mirror plate with respect to the fixed member toalign the beam will not change the length of the cavity.

In the third aspect of the subject invention, an improved assembly isdisclosed for automatically compensating for changes in length of thesupport bar. In the preferred embodiment, the assembly is designed formounting a pair of mirrors utilized for folding the path of the beamwithin the resonator. The assembly includes an elongated beam having acenter section mounted to the end of the bar. The opposed ends of thebeams are defined by flexure members having a parallelogramconfiguration. When the flexure members are rotated relative to thecenter section, the opposed ends will translate but remain parallel toeach other and the support bar due to the deformation properties of aparallelogram. The opposed ends of the flexure members are used asmounting surfaces for the fold mirrors.

The assembly further includes a clamp connected to the bar. A biasingmember is connected to the clamp. The biasing member is formed from amaterial having a coefficient of thermal expansion greater than the barsuch that any variations in temperature will affect the length of thebiasing member to a greater extent than the bar. A means is provided forconnecting the biasing member to the flexure members. In the preferredembodiment, this means is defined by a lever connected to the ends ofthe flexure members.

In accordance with the subject invention, when a temperature variationoccurs that affects the length of the bar, the simultaneous change inlength of the biasing member will cause each flexure member to rotatewhile the opposed ends thereof, having the mirrors mounted thereon, willtranslate in a direction opposite to the length of change of the bar. Bythis arrangement, the total length of beam path will remain unchanged.

Further object and advantages of the subject invention will becomeapparent from the following detailed description taken in conjunctionwith the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan layout view of a laser incorporating the improvedmounting assemblies of the subject invention.

FIG. 2 is a perspective layout view of the laser shown in FIG. 1.

FIG. 3 is a cross sectional view, taken along the line 3--3 in FIG. 2,of a portion of the kinematic mounting for a bar.

FIG. 4 is a side view of the kinematic assembly used for mounting thebar.

FIG. 5 is a cross sectional view, taken along the line 5--5 of FIG. 2,of the gimbal mounting for a laser mirror.

FIG. 6 is a perspective view of the one piece cylindrical gimbal ring ofthe subject invention.

FIG. 7 is a cross sectional view, taken along the line 7--7 in FIG. 6,of the cylindrical ring of the subject invention.

FIG. 8 is a cross sectional view, taken along the line 8--8 in FIG. 6,of the cylindrical ring of the subject

FIG. 9 is an enlarged, exploded, perspective view of the assembly forautomatically compensating for the elongation of support bar due totemperature.

FIG. 10 is a top plan view, partially in section, taken along the line10--10 in FIG. 2 of the assembly shown in FIG. 9.

FIG. 11 is a view similar to FIG. 10, showing the expansion of thebiasing member which in turn causes the flexure of the beam therebymaintaining the total path length of the laser beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 illustrate the various assemblies of the subject inventionused in conjunction with the laser wherein the length stability of theresonant cavity is important. It is expected that each of the assembliesdescribed herein will find use in other lasers. Their description,however, in terms of a specific laser example, is intended to aid in anunderstanding of the individual components.

As shown in FIGS. 1 and 2, a laser gain medium 12 is shown foramplifying light 14 passing therethrough. The gain medium assembly istypically mounted to the frame 16 of the laser device. The gain medium12 sits within a resonant cavity, defined by two end mirrors mounted inassemblies 20A and 20B. In order to reduce the overall linear length ofthe resonant cavity, a pair of fold mirrors 30A and 30B are provided forreversing the direction of the beam within the cavity. The path of thebeam within the cavity therefore extends from mirror 20A down to mirror30A across to mirror 30B and back to mirror 20B. As noted above, it isdesirable to make sure that the total path length of this beam remainsconstant for various applications.

One approach to minimize changes in the path length is to mount thevarious optical components on a stable bar 32 having a low coefficientof thermal expansion. In the preferred embodiment, the bar is formedfrom Invar. The Invar bar is mounted to the frame 16. In accordance withthe subject invention, the bar 32 is mounted to the frame using animproved kinematic assembly. As will be described in greater detailbelow, the kinematic assembly includes a pair of brace members 40A and40B and a clamp member 42. The mirror mounts 20A and 20B at the ends ofthe resonant cavity are typically provided with the ability to adjustthe angles of the mirrors to align the laser beam within the cavity. Aswill be described below, the mounting assemblies 20A and 20B arespecially designed to allow adjustment of the angle of the mirrorswithout altering the length of the cavity.

In the third aspect of the subject invention, the fold mirrors 30A and30B are mounted on an assembly 50 which is designed to automaticallycompensate for changes in length of the bar 32. While the low thermalcoefficient of the expansion of the Invar bar eliminates gross changesin the size of the resonant cavity, even small changes can affect theperformance of certain lasers. The assembly 50 is designed toautomatically compensate for these small changes.

Turning now to FIGS. 3 and 4, the kinematic mounting for the Invar barwill be discussed in greater detail. The mounting assembly includes apair of brace members 40A and 40B mounted to the frame 16. The bracemembers are intended to act as a gravitational support for the Invar bar32. The configuration of the brace members must allow for limitedmovement of the bar both rotationally and longitudinally. In thepreferred embodiment of the subject invention, each brace member 40includes upper and lower sections 102 and 104. These sections areaffixed together by a pair of screws 106 which are spring-loaded bywashers 108. The screws pass through the upper member 102 and arethreaded into the lower member 104. By this arrangement, a certainamount of spring tension can be placed on the bar.

In the illustrated embodiment, each brace member 40 is provided with ancircular opening 110 for receiving the circular bar 32. The circularopening 110 is provided with a bearing means. More specifically, atthree points spaced about the opening 110, a cubical recess 112 isprovided into which ball bearings 114 are received. A pair of hardenedtungsten carbide disks 116 are mounted on either side of the ballbearing. The ball bearing 114 and pads 116 are fitted within a foamcylinder 118. This bearing arrangement provides a low wear, low frictioninterface between the bar 32 and the brace member 40. As illustrated inFIG. 2, the two brace members are spaced apart on the frame by an amountapproximately sixty percent of the length of the bar.

The kinematic support further includes a clamp 42 which is securelymounted to the bar 32, adjacent one of the brace members 40A. In theillustrated embodiment, clamp 42 is made up of upper and lower portions120 and 122 connected by a pair of screws 124. These screws aretightened to securely attach the clamp to the bar.

Clamp 42 is connected to the brace member 40A. This connection must beof the type which will only constrain the remaining degrees of freedomof motion allowed by the brace members alone. No additional constraintsmay be added by this connection. A variety of mechanical means could beused to achieve this result and limit the remaining longitudinal androtational movement of the bar. In the preferred embodiment, theconnection means is defined by the engagement of a conical nose and achamfered slot.

As shown in FIG. 4, the conical nose 130 is formed on the clamp 42 andis received in slot 132 formed in the brace 40A. The sides of slot 132are chamfered at 134. The elongated slot extends vertically in the brace40A which is along the radius of the circular cross section of the Invarbar. In the preferred embodiment, the head of the nose 130 is drawn intothe slot by a screw 138. As illustrated in FIG. 4, the screw 138, whichis countersunk in the clamp 42, passes through the nose to the otherside of the brace. The end of the screw is affixed by a nut 140 about aspring washer 142.

The kinematic mounting assembly described herein removes all degrees offreedom of motion of the bar with the minimum number of constraints. Inthis manner, any force which is generated in the frame 16 by outsideforces will not be transmitted to the bar.

Turning now to FIGS. 5 through 8, there is illustrated the first of twoimproved mirror mounting assemblies of the subject disclosure. Themirror mount 20 includes a unique, one-piece cylindrical gimbal pivotring 210, shown best in FIG. 6. The use of this simple pivot ringprovides a low cost and effective mounting system for maintaining thelength of the resonant cavity of the laser while allowing the angle ofthe mirror to be adjusted.

The cylindrical ring 210 is provided with four mounting arms 212A-D,located about the circumference of the ring at 90° intervals. One pairof opposed mounting arms 212A and 212C, project towards one end face ofthe ring while the remaining two arms project towards the other end ofthe ring. Each arm is defined by a pair of slots 214 machined into thering. Each slot 214 terminates in a hole 216. The material of the armbetween the pairs of holes 216 defines an area of flexure 218 aboutwhich the ring can pivot. The flexure areas 218 are all arranged to bein a common radial plane. The straight line between each pair of opposedflexure areas defines a pivot axis. The intersection of these two axeslies in the center of the ring and defines the pivot point of the ring.

As illustrated in FIG. 5, one pair of arms 212 are mounted to a fixedplate 232 which is in turn connected to the Invar bar 32. The connectionis made through a pair of screws 234 passing through the fixed plate andinto the ends of the mounting arms. The remaining set of mounting armsare used to affix a mirror plate 240 via screws 242. Mirror plate 240includes a mirror support 244 projecting within the interior of the ring210. The end of mirror support is used to mount a mirror 246. In thepreferred embodiment, the face of mirror 246 is located substantially inthe radial plane defined by the flexure areas 218. This position,however, is not essential, particularly if one insures that the path ofthe incoming beam crosses the pivot point of the ring.

In accordance with the subject invention, the mirror plate 240 isadjustable with respect to the fixed plate 232. As illustrated in FIG.5, one or more springs 248 are mounted between the mirror plate 240 andthe fixed member 232. In addition, a plurality of adjustment screws 250are provided for pivoting the plate. Screws 250 are threaded throughopenings in the plate and abut the fixed member 232. As the screws arerotated, the plate will tend to rotate with respect to the fixed member232 about the pivot point defined by the flexure regions 218 in thegimbal ring 210. In use, the angle of the mirror can be adjusted toalign the laser beam without altering the length of the cavity.

Turning now to FIGS. 9-11, the third aspect of the subject inventionwill be discussed. More specifically, a mounting assembly 50 isdisclosed for compensating for changes in length in the bar 32 due totemperature. The assembly 50 includes an elongated beam 302 having acenter section 304 which is connected by four screws 306 to one end ofthe Invar bar 32. The beam 302 further includes a pair of opposedflexure members 310 hingedly connected thereto. Each of the flexuremembers can be thought of as having a parallelogram configuration. Theconfiguration of each flexure member is analogous to a four bar linkage.This configuration is achieved by providing a plurality of slots orhinges 312 in the beam. The slots are arranged such that opposed sidepieces of the member are of the same length. As long as the lattercriteria is met, when the flexure members are rotated with respect tothe center section 304, the end pieces 316 will remain parallel to eachother and to the support bar. The parallel relationship is necessary toinsure that the angle of the beam is not changed. A pair of mirrors 320are mounted to the ends 316 of the flexure members 310.

Assembly 50 further includes a clamp 330 mounted to the Invar bar 32,spaced from beam 302. A biasing member 332 is connected to the clamp330. Biasing member is formed from a material that has a coefficient ofthermal expansion greater than that of the Invar bar. Accordingly, anyvariations in temperature will affect the length of the biasing memberto a greater extent than the Invar bar. The expansion of the biasingmember is coupled to the flexure members 310 via a lever 340. Lever 340is defined by an elongated bar, the opposed ends of which are connectedto the flexure members 310 via screws 342. The center of lever 340 isnot connected to the center section 304 of beam 302. Lever 340 includesa pair of slots 344 which define hinges about which the lever can flex.

In use, the position of the fixed members 232 (FIG. 2) carrying the endmirror assemblies 20 are adjusted to set the desired length of thecavity. Any temperature variations which occur thereafter will effectthe length of the Invar bar. Without the present mechanism, thistemperature variation will cause mirrors 320 to be moved either closerto or farther from the end mirrors thereby changing the length of thecavity. The subject invention automatically compensates for this changein length of the bar.

This result is illustrated in FIG. 11. More particularly, a temperaturevariation which would cause the bar to expand will cause the biasingmember to expand even more. This expansion will place a force on thelever 340 in the direction of arrow A. The force on the lever will causethe flexure members to rotate in the directions shown by arrows B inFIG. 11. Because of the four bar or parallelogram configuration of theflexure members 310, the ends 316 thereof will move in a directionopposite to arrow A while remaining parallel to each other and theiroriginal orientation. The direction of movement of the ends will be in adirection opposite to direction of expansion of the end of the Invarbar. The extent of the movement of the flexure members can be controlledby the location at which the biasing member 332 is connected to theclamp 330. This adjustment can be made through a set screw 350.

Although not illustrated in FIG. 11, the assembly 50 will react and workwhen the ambient temperature is reduced. In this case, biasing member332 will contract, pulling on lever 340 and causing the flexure membersto rotate in the opposite direction of that shown in FIG. 11. While theillustrated embodiment shows a pair of flexure members 310, the subjectconcept could be implemented with a single flexure member. For example,one of the two mirrors 320 could define the end of a resonant cavity andactually include a mirror mount similar to those described above at 20A.It is intended that the scope of the subject invention includes anassembly with a single flexure member.

While the subject invention has been described with reference to thepreferred embodiments, other variations could be made, by once skilledin the art, without varying from the scope and spirit of the subject asdefined by the appended claims.

I claim:
 1. An assembly for adjustably mounting a mirror to a fixedmember comprising:a pivot member having a central opening and twoopposed end faces, said pivot member further including four mountingarms formed therein and spaced apart by about 90 degrees, with one pairof opposed mounting arms projecting towards one of the two end faces andwith the other pair of opposed mounting arms projecting towards theother end face and with each arm having a region of reduced widthcapable of flexure, and with all of said flexure regions lyingsubstantially in one radial plane intermediate the opposed end faces ofsaid pivot member, and with one pair of said mounting arms beingconnected to said fixed member; a mirror plate connected to theremaining pair of mounting arms of said pivot member, said mirror plateincluding mirror holder for mounting a mirror thereon; and means foradjusting the mirror plate with respect to the fixed member about saidflexure regions of said mounting arms.
 2. An assembly as recited inclaim 1 wherein said pivot member is a cylindrical ring.
 3. An assemblyas recited in claim 1 wherein said means for adjusting the mirror plateincludes a plurality of screws threaded though said mirror plate andabutting said fixed member.
 4. An assembly as recited in claim 1 whereinsaid mirror holder projects within the interior of said pivot member. 5.An assembly as recited in claim 1 wherein said mirror holder projectswithin the interior of said pivot member in a manner such that themirror will lie substantially in said radial plane defined by saidflexure regions of said mounting arms.